title: Nucleotide-assisted [Fe4S4] redox state interconversions of the Azotobacter vinelandii Fe protein and their relevance to nitrogenase catalysis.
abstract: The [Fe4S4] cluster of the nitrogenase Fe protein from Azotobacter vinelandii can exist in three redox states: oxidized [Fe4S4](2+), dithionite reduced [Fe4S4](1+), and two forms of the all ferrous [Fe4S4](0), S = 4 and 0. Operation of the [Fe4S4](2+)/ [Fe4S4](1+) redox couple transfers one electron to the MoFe protein during catalysis with hydrolysis of two MgATPs. In contrast, the [Fe4S4](2+)/[Fe4S4](0) couple transfers two electrons per binding event, accompanied by hydrolysis of only two MgATPs. Both reactions occur at nearly identical rates even though the number of electrons transferred differs by a factor of 2. MgATP and MgADP facilitate interconversion of the three redox states: 2[Fe4S4](1+) + 4 MgATP = [Fe4S4](2+)(MgATP)2 + [Fe4S4](0)(MgATP)2, as demonstrated by the MgATP reaction. This reaction was investigated as a possible precursor reaction to provide two electrons in the form of [Fe4S4](0)(MgATP)2 for delivery to the MoFe protein to then conduct a two-electron substrate reduction. However, experiments showed that this disproportionation reaction, which readily occurs, was not viable during nitrogenase catalysis utilizing the [Fe4S4](1+) cluster state. The known cooperative behavior of the Fe protein in the [Fe4S4](1+) state taken together with a measured turnover potential of -460 mV with an n = 2 value, suggest a gating process on the MoFe protein involving a two electron step.
authors.affiliation: Department of Chemistry and Biochemistry, Brigham Young University , Provo, Utah 84602, United States.
authors.forename: Deloria
authors.initials: D
authors.lastname: Jacobs
authors.forename: Gerald D
authors.initials: GD
authors.lastname: Watt
mesh_terms: D000244:Adenosine Diphosphate / Q000737:chemistry; D000255:Adenosine Triphosphate / Q000737:chemistry; D016948:Azotobacter vinelandii / Q000737:chemistry*; D002384:Catalysis; D007506:Iron-Sulfur Proteins / Q000737:chemistry*; D009591:Nitrogenase / Q000737:chemistry*; D010084:Oxidation-Reduction
chemical_list: D007506:Iron-Sulfur Proteins; D000244:Adenosine Diphosphate; D000255:Adenosine Triphosphate; D009591:Nitrogenase